Remove core/platform/win

This CL moves the files to platform/win.

TBR=eseidel@chromium.org
BUG=297477

Review URL: https://codereview.chromium.org/26113005

git-svn-id: svn://svn.chromium.org/blink/trunk@159156 bbb929c8-8fbe-4397-9dbb-9b2b20218538